r600: amend space check for chips older than EVERGREEN
evergreen_emit_atomic_buffer_setup_count is only called if chip >= EVERGREEN
otherwise atomic_used_mask is left uninitialized when unconditionally used by r600_need_cs_space so it might want more space than needed
fix this by always initializing atomic_used_mask
Fixes: 32529e60 ("r600/eg: rework atomic counter emission with flushes")